This invention relates to a cross-polarization interference cancellation system for use in a microwave digital radio communication network which carries out communication by the use of polarized waves having planes of polarization orthogonal to each other.
Recent attention has been directed to an orthogonal polarization digital radio communication network which carries out communication through a transmission medium by the use of a pair of polarized waves which are orthogonal to each other and which may be, for example, either a pair of a vertical polarized wave and a horizontal polarized wave or a pair of a right-handed polarized wave and a left-handed polarized wave. At any rate, it is possible in such an orthogonal polarization digital radio communication network to effectively utilize frequencies necessary for transmission of information signals because the polarized waves of the same frequency are individually used to transmit the information signals different from each other. Herein, each of the information signals is assumed to be produced at a predetermined transmission rate.
In such an orthogonal polarization digital radio communication network, cross-polarization interference often takes place between the polarized waves, for example, from the vertical polarized wave to the horizontal polarized wave due to that anisotropy of the transmission medium which might result from rain or the like.
A conventional cross-polarization interference cancellation system has been proposed so as to cancel such cross-polarization interference and basically comprises a cross-polarization interference cancellation circuit and a demodulator circuit. More specifically, the cross-polarization interference cancellation circuit is supplied with first and second input signals which stem from polarized radio waves having planes of polarization orthogonal to each other and which may be, for example, intermediate frequency signals. In this event, each of the first and the second input signals might be subjected to cross-polarization interference and might therefore include interference components due to the cross-polarization interference, respectively. In order to cancel such interference components, the cross-polarization interference cancellation circuit comprises first and second cancellation units which are operable to cancel the interference components included in the first and the second input signals, respectively, and which are similar in structure and operation to each other. Each of the first and the second cancellation units comprises a delay circuit for delaying each of the first and the second input signals to produce a delayed input signal, a transversal filter circuit for reproducing the interference component included in the other input signal to subtract the interference component from the delayed input signal to produce an interference free signal, and a control signal generator for generating a plurality of control signals to deliver the control signals to the transversal filter circuit. The transversal filter circuit comprises a plurality of delay units each of which has a delay time equal to the transmission rate.
On the other hand, the demodulator circuit comprises first and second demodulators supplied with the interference free signals from the first and the second cancellation units to demodulate the interference free signals into demodulated signals. The demodulated signals fall within a selected one of first through fourth quadrants when they are represented on a phase plane. In addition to the demodulated signals, each of the first and the second demodulators also produces a set of error signals and quadrant detection signals representative of the selected quadrant.
It is to be noted that each of the error signals and the quadrant detection signals are detected by the use of a clock pulse which has a clock frequency equal to the transmission rate.
With this structure, each of the delay units in the transversal filter circuit is operable at the clock pulse of the clock period sent from the other cancellation unit. In addition, the error signals and the quadrant detection signals are also detected by the clock pulse.
Accordingly, when a time difference of T/2 exists between the interference component and each input signal from which the interference component is cancelled, an ability of the cross-polarization interference cancellation is seriously reduced.
Alternatively, another conventional cross-polarization interference cancellation system is disclosed in an article which is contributed by B. Lankl et al to 1988 IEEE, pp. 1355-1361, and which is entitled "Cross-polarization interference in the presence of delay effects". With this structure, the error signals are discriminated and reproduced after coherent detection by the use of a clock pulse which has a clock frequency equal to twice the transmission rate. This shows that the error signals are extracted at transition points, although the quadrant signal is reproduced by a clock pulse having a clock frequency equal to the transmission rate. Such extraction of the error signals at the transition points objectionably raises a probability of extracting an erroneous information signal and brings about instability of operation.